Any attack that can instantly kill the person in a singular hit that is either definitive, or outright cause a character to die, or be permanently incapped. There's always a shot to dodge Fran's phantasm if you're able to somehow teleport or whatever, or if you can move quick enough and you see Mordred's attack coming you can dodge that. They're big brutal hits but they don't instantaneously declare the individual dead the second they're called.

In DnD terms its would be like banning power word kill.

https://www.dndbeyond.com/spells/power-word-kill